This invention relates to expanding gate valves and more particularly to an expanding gate valve provided with a dual stage pneumatic-powered actuator.
With single stem valves which handle very high pressures, the large forces needed to operate the valve preclude the use of any manual means, such as a handwheel, and require the provision of an automatic operator. Automatic operators are also widely used in applications for operating a valve in automatic response to sensing of some changed condition as, for example, with safety valves which are incorporated in safety systems.
Heretofore, the use of fluid-powered actuators with gate valves of the expanding gate type has presented many problems. Variances in the sealing forces between expanding gate valves, even those of the same kind, usually rules out the adaptability of an actuator with a uniform thrust force for a variety of valves wherein such variant characteristics are the general rule. Also, the very large sealing forces of these valves require a very powerful actuator thrust to break the "seal" and such a powerful thrust can serously damage the valve components, such as the seal rings or the gate and segment members. A very large spring force is also needed to return the actuator piston upon release of fluid pressure from the actuator and these significantly increase the size, weight and expense of the actuator. Nevertheless, the handling of very high fluid pressures of 20,000 p.s.i. or more, as is characteristic of deep gas and oil wells, has made it very desirable that only gate valves of the expanding gate type be used, and particularly so in safety valves for very high pressure systems. As opposed to hydraulic actuators, pneumatic actuators are often times used where quick response times are needed, where reduction of the flammability hazard is important, or where pneumatic power is readily available.
In the design of actuators for expanding gate valves, the piston must be sized so minimum fluid pressure to the actuator is able to overcome the valve's gate drag plus the pressure which acts across the cross section of the stem to oppose the actuator thrust. At approximately one third of the actuator stroke distance, the gate ports and valve conduct flow passage come into communication so that pressurized fluid in the valve body cavity is vented to the flow passage. As this happens, both the gate drag and the opposing stem force are lost. If the actuator fluid is hydraulic, a small displacement of the actuator piston suffices to lower the fluid pressure driving the actuator to substantially zero so that the valve completes its stroke, as in moving from closed to open position, at a relatively slow speed determined by the flow rate of the actuator pressure source fluid and the actuator orifice inlet.
In applications where pneumatic actuators are used to control the operation of gate valves, the instant of occurrence of lost gate drag and opposing stem face during the actuator stroke allows the actuator fluid, a gas, to rapidly expand. Because the gas is compressible, the actuator piston moves a distance much greater than the valve stroke before the hydraulic actuator pressure is significantly reduced. The result is the connected "train" of actuator piston, stem and valve gate assembly moves with such greater velocity and momentum as to "slam" against the valve stop which causes the rapid expansion and very tight wedging of the gate and segment elements of the valve gate assembly in its expanded sealed condition. In addition to the likelihood of valve damage, a further disadvantage is that a very large force is required to break the "wedge" to permit further operation or "fail-safe" operation of the valve. One solution to this problem is disclosed in U.S. patent application Ser. No. 568,460, filed Jan. 5, 1984 now U.S. Pat. No. 4,535,967, which shows an expanding gate valve operated by a piston type pneumatic-powered actuator which includes a hydraulic choke means to reduce the "slam" effect. There is, of course, the possibility of leakage of hydraulic fluid from the hydraulic choke, especially over a long period of time, such that a totally pneumatic actuator system may be preferred. The provision of a hydraulic choke means also increases the number of parts and the complexity of the actuator.
Accordingly, it is an object of the invention to provide a simple and reliable pneumatic actuator for operation of a gate valve of the expandable gate type.
It is another object to provide a dual piston pneumatic-powered dual piston actuator for operation of a gate valve of the expandable gate type wherein actuator stem velocity is controlled to substantially constant velocity by restricting the flow of actuating fluid to the actuating piston at a time during the actuator stroke when the valve gate ports first come into communication with the flow passage through the valve.
A further object is to provide a pneumatic-powered dual stage actuator for actuating the control element of a mechanical device wherein actuator stem velocity is controlled by sudden restriction of air flow to the actuating drive piston means in the latter stage of actuator thrust.